Self-luminous signs and indicators have been in use since early in this century and have experienced numerous improvements over the intervening years. The early uses of self-luminosity employed radium as the activator for a phosphor; however, radium constituted a health harazd from its "hard" radiation and was abandoned. In more recent times a number of radio-isotopes have been developed and produced, which serve to activate phosphors to luminescence. Depending upon the choice of isotope, one may obtain alpha, beta or gamma radiation and it has been found that alpha and gamma radiation are hazardous to health, leaving the beta radiators as the safe type for self-luminescence devices. by definition, the beta radiators emit electrons which are relatively heavy particles and exhibit less velocity. This type of radiation will not penetrate a thin glass wall such as is employed in my invention; however, it is effective in cause phosphors to luminesce. Among the beta radiating isotopes, I have selected Tritium as the activator for my device. Tritium exhibits a half-life of 12.5 years, which it quite adequate for the purpose intended. Other isotopes might be used; however, some have small amounts of "hard" radiation and exhibit differing half-lives, such as:
Promethium.sup.147, having a half-life of 2.7 years, PA1 Thallium.sup.204, having a half-life of 3.6 years and PA1 Krypton.sup.85, having a half-life of 10.0 years; however, this isotope yields approximately 0.5% of its radiation in the form of gamma rays. PA1 (a) Light being obstructed by the phosphor and the radioactive substance being chemically combined to become a solid. PA1 (b) Light being obstructed or attenuated by having to pass through a layer of phosphor to become visible. PA1 (c) Light being limited by only one side of the phosphor particles being exposed to the radiation.
Other have made various forms of self-luminous devices; however, these have suffered from lack of efficiency for any of the followings causes:
A further problem with some of the previous devices has been that the phosphor was combined with a binder to allow a film coating on the inside of a glass envelope which contained the radioactive gas. In this instance, not only did the film attenuate the light, but the binder deteriorated with time due to its exposure to the radiation.
Others have made self-luminous paints, wherein the radioactive gas was converted to a solid by chemical combination with a transparent polymer, which was then deposited on phosphor crystals. In this instance, exposure to its own radiation resulted in the tritiated polymer losing gas and the tritiated gas compounds readily diffuse through the polymer, thus resulting in a radiation hazard, as well as to degrade the transparency of the polymer.
Work in the area of self-luminous signs has been done by such companies as American Atomics, Inc., Self Powered Lighting, Inc. and by the Oak Ridge National Laboratories (ORNL). See U.S. Pat. No. 4,383,382 of Self Powered Lighting, Inc. In addition, the 3M company has done considerable work with self-lumination; however, their means invole the hazard and light attenuation problems described above.
NASA's Jet Propulsion Laboratory has done work with the confinement of atomic waste materials in glass envelopes and in a manner similar to that described herein; however, they employed a standard method of forming glass spheres and they were not concerned with self-luminescence. No phosphors were involved with their work.